Broadband amplifiers require a DC bias that will operate effectively over their entire operational frequency ranges. Some broadband amplifiers, such as traveling wave amplifiers that are used in telecommunications switching networks, need to operate from the kilohertz range to the tens or hundreds of gigahertz range. Some broadband amplifiers use bias resistors and current sources to provide DC bias to the amplifiers. One problem with using bias resistors and current sources is that the current traveling through the resistors results in resistive power loss proportional to the square of the current and the resistance of the resistor, reducing the power efficiency of the amplifiers.
Because power efficiency is a key design constraint of broadband amplifiers, DC bias circuits that utilize resistor and current source designs are not optimal. The highest efficiency broadband amplifiers use inductor based bias chokes. An inductor based bias choke consists of a voltage source and an inductor. Ideally, the inductor should appear as a short circuit to direct current to allow the DC bias current to reach the amplifier without high resistive losses. The inductor should appear as a very high impedance to alternating current (AC) signals over the operational frequency range of the amplifier so that the impedance of the inductor does not adversely affect the operation of the amplifier. For example, capacitance between the windings of an inductor causes the inductor to self-resonate at its self-resonant frequency. If the self-resonant frequency of the DC bias inductor is within the operational frequency range of the amplifier, the DC bias circuit will appear as a short circuit to ground at the resonant frequency. A short to ground in the DC bias circuit of a broadband amplifier changes the impedance seen looking into the output terminals of the amplifier and adversely affects its operation.
One type of inductor bias choke that operates over wide frequency range requires large, expensive, conical wound ferrite core in order to present a large inductive impedance with reduced self-resonance caused by parasitic capacitance of the windings. Requiring conical wound ferrite core inductors to present the proper impedance increases the cost of broadband amplifiers.
Accordingly, there exists a need for a broadband amplifier with a tapered attenuation network for mitigating the effects of bias inductor self-resonance.